In automobiles with an internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which is pumped a liquid. This liquid is typically a mixture of water with antifreeze. The fluid moves in a closed system from the radiator to the engine, where it conducts heat away from the engine parts. It then flows through a thermostat (controlling the rate of flow) back to the radiator, where it is cooled again by convection with the air. This process cools the entire engine. Some engines have an additional oil cooler; a separate small radiator to cool the motor oil. Turbo charged engines may have an intercooler.
In buildings, a radiator is a heating device, which is warmed by hot water being pumped into it from a water heater. Hot water enters at one end and rises to the top of the radiator. As it gives out its heat, it cools and sinks to the bottom of the radiator and then is forced out of a pipe at the other end. If there is air trapped inside the radiator, then the water cannot rise to the top, and only the bottom area gets hot. A bleed like near the top of the radiator allows the trapped air to be 'bled' from the radiator, and thus restore correct operation. Often radiators located on upper floors will accumulate more air than ones on lower floors as the air will tend to rise to the topmost point in the system. These may have to be bled more often. Usually radiators are bled once or twice per season, or as needed. The air near a radiator is heated and produces a convection current drawing in cold air to heat. If set up improperly radiators can make loud banging noises like someone hammering on the pipes. This is due to the pipes rubbing on surrounding surfaces while expanding and contracting due to heat changes. Stereotypical radiators are no longer common in new construction. The current trend in radiator heating is towards floor heating, where the hot water is circulated under the entire floor of each room in a building.